1. Field of the Invention
The present invention generally relates to a parallel-to-serial signal converting apparatus, and particularly relates to a parallel-to-serial signal converting apparatus, when used with an image displaying system in which a serial bit signal representing an image to be displayed on a display unit of the image displaying system is transmitted in the form of low frequency parallel bit signals, adapted to convert the parallel bit signals into a serial bit signal representing the image to be displayed.
2. Description of the Prior Art
When video data of a high frequency serial bit signal representing an image to be displayed on a display unit in an image displaying system are transmitted, in order to prevent various troubles such as radiation noises or the like due to high frequency waves from occurring, the video data of the serial bit signal are converted into video data of parallel n-bit signals of a low frequency and the thus converted parallel video data are transmitted. In order to obtain a bit shift clock signal for determining the timing when such parallel video data are converted into the serial video data representing the image, a character clock signal is conventionally transmitted together with the parallel signals, for example, as disclosed in JP-A-61-233779. In this prior art, however, there has been a problem in that it is necessary to provide a PLL circuit for obtaining a bit shift clock signal from the character clock signal and therefore a display unit becomes complicated and expensive.
In the prior art disclosed in JP-A-61-105587, on the other hand, a plurality of shift registers are provided for converting parallel video data into serial video data. In this prior art, however, there has been a problem in that it is necessary to provide a selector for changing-over the outputs of the plurality of shift registers and to use a high-frequency bit shift clock signal for controlling the selector, so that the threshold frequency of a video signals is determined by the highest operating frequency of circuit elements other than the shift registers.